Has the differential movement been accounted for?

Once the brickwork is built, the frame and the brickwork start moving in opposite directions. A concrete frame shortens and its slab edges sag over time. Clay brickwork expands, and some of that expansion never comes back. Differential movement is the gap between those two behaviours, and on a framed building with brick cladding it all ends up in one place: the horizontal movement joint under the masonry support.

That joint is usually drawn as a single line on an elevation with a note beside it. It is rarely calculated. This post is about what that line has to absorb, why nobody tends to add it up, and what happens when the sum comes out bigger than the joint.

Part three asked where the frame actually is when the brickwork starts. This one asks where it goes afterwards.

The question

Has the differential movement been accounted for?

Not “is there a movement joint on the drawing”. There almost always is. The question is whether anyone has worked out how much movement that joint needs to take, from which sources, and whether the joint as drawn and as built can actually take it.

Why it goes unasked

Because every individual number on the project is right.

The structural engineer designs the frame to its deflection limits. The brick manufacturer can tell you how much their product is likely to expand. The architect draws a movement joint under every support, as the standards ask. The masonry support designer checks the angle for its own deflection under the brick load. The bricklayer leaves a gap at the head of each panel.

Each of those is done properly, by the person responsible for it. What nobody owns is the combined movement assessment. It is not simply a case of adding every number together. Some movements happen before the brickwork is installed, some afterwards, and not all act in the same direction. The important question is how much relative movement will actually close the joint once the masonry is in place. The joint sits at the interface between the frame, the support and the brickwork, which is exactly the kind of place where, as part one of this series argued, responsibility tends to evaporate.

There is also a habit problem. A movement joint gets drawn at a standard width because the last project had one at that width, and the detail travels from drawing to drawing without anyone checking it against this building, this frame and this brick.

What differential movement puts into the joint

The NHBC Standards are unusually direct about this. Clause 6.1.16 asks for a horizontal joint that can accommodate all the movement anticipated at it, including deflection of the substrate and of the angle support system, and that allows for the limited compressibility of both the filler and the sealant. It adds that horizontal movement joints should take at least 1mm of movement per metre of continuous clay masonry below.

That is a list, and it is worth unpacking, because each item comes from a different person.

Brick expansion. Clay brickwork grows over its life. The Brick Development Association’s 2023 guide puts unrestrained growth at roughly 1mm per metre from combined thermal and moisture movement, notes that some clays move more, and advises checking with the manufacturer. Change the brick and this number can change.

Frame shortening. A concrete frame shortens under its own weight, then keeps shortening as it dries and creeps under sustained load. Over two storeys between supports, that adds to the closing of the joint from above at the same time as the brickwork closes it from below.

Slab edge deflection. The slab edge between columns sags over time, and the support angle fixed to it goes with it. What matters for the joint is not the total deflection but the part that happens after the brickwork below has been built. The Concrete Society’s Technical Report 58 is clear that deflection needs to be considered after cladding is installed, not just before, and that calculated long-term deflections carry real uncertainty. Flat slabs without edge beams are the condition NHBC’s technical guidance singles out.

Support deflection. Masonry support angles and brackets deflect under the weight of brickwork, depending on their design and loading. However, only additional deflection occurring after the movement joint is formed contributes to its closure. The masonry support designer must establish this allowance and coordinate it with the structural engineer’s movement assessment.

As-built variation. The joint is built at whatever width the frame and the brickwork allow on the day. If the frame is low at a given bay, the joint is tight before any movement has happened. This is where part three’s tolerance question and this one meet.

Usable compression. The BDA recommends joint fillers capable of compressing to approximately half their original thickness. However, a 10mm joint does not automatically provide 5mm of usable movement. Actual capacity depends on the filler, sealant and construction tolerances. Joint widths must be calculated against anticipated movement rather than assumed from standard details.

The illustrative 10mm is deliberate, because it is the width most often drawn. It is a common minimum, not a calculated answer, and once the filler’s compressibility and any as-built tightness are taken off, what remains has to cover everything in the list above.

Spacing joints for differential movement

Width is half the question. Spacing is the other half.

The BDA guide says that where the frame is reinforced concrete, the full potential differential movement between frame and clay masonry has to be allowed for, which may mean horizontal joints at every two storeys. For other buildings it suggests support at no more than every third storey or 9m, whichever is less, and allows the outer leaf to run its full height on buildings up to four storeys or 12m.

The support levels are therefore a movement decision as much as a structural one. Fewer support levels means less steel, but more brickwork growing into each joint and more frame shortening between them. That trade-off should be made deliberately.

Vertical movement joints need the same attention. The BDA guide recommends clay brickwork joints at roughly 10–12m and notes PD 6697’s limit of 15m. But movement joints in the brickwork and gaps between masonry support angles serve different purposes. Support angles can continue across vertical movement joints in the brickwork, provided the detailing allows the masonry to move independently. The gaps between individual support trays, typically 10mm in FIRMA systems, accommodate installation tolerances and thermal expansion of the steel and do not necessarily need to align with the brickwork joints. Structural movement joints are different. The masonry support must not bridge movement joints in the primary frame unless a specifically engineered detail allows for it. The important thing is that all three are coordinated, not that every joint lines up.

Steel frames change the mix rather than removing it. There is less shrinkage and creep than in concrete, but edge beams still deflect under the cladding and floor loads, and the brick still grows.

What happens when nobody asks

Usually, nothing. For a while.

Differential movement is slow. Brick expansion and frame creep both continue for years, so a joint that is too small at handover often looks fine. It closes gradually. Once it is closed, the brickwork below starts carrying the load of the panel above, which it was never designed to do.

The failure modes are well documented. Christine Beall’s Masonry Design and Detailing describes them: bowing of the outer leaf, horizontal cracking at the support level, vertical cracks near corners, and spalling where stress concentrates, at window heads and at the toe of the angle. Research from Canada’s National Research Council has also recorded support angles rotating as the frame moves down relative to the cladding.

A typical sequence runs something like this. The joint is drawn at a standard width. On site, the frame at some bays is lower than drawn, so the joint is built tight. A few mortar snots and a hard packer end up in it before it is sealed. Over the next few years, the brick grows and the slab edge creeps. The joint has nothing left to give, and a horizontal crack appears along a course just below the angle, often after the defects period has ended.

At that point, every party can show their own work was correct. The frame met its deflection limits. The brick met its standard. The angle met its design. The joint was on the drawing. It is part one’s problem again, in a different form.

The remedy is expensive. Cutting out and rebuilding brickwork at height, reforming joints, sometimes adjusting or replacing support, with scaffold, access and an occupied building to work around. All for a number nobody added up at Stage 4.

What a good answer looks like

None of this is complicated. It is a short calculation and a conversation between three or four people who would otherwise each assume someone else had done it.

Differential movement and value engineering

Most of the inputs to the movement budget are also the things that get changed to save money. A different brick may expand more. Fewer support levels means larger joints. A thinner slab may deflect more. A cheaper filler may compress less. Each change can be sensible on its own, but each one moves the budget. Part five asks whether value engineering savings are real, and differential movement is one of the places where a saving on paper can quietly turn into a cost later.

What this means for you

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References

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